Methods of making single mode optical fibers with reduced bend sensitivity and catastrophic bend loss

ABSTRACT

Described are multi-tube fabrication techniques for making an optical fiber that is relatively insensitive to bend loss and alleviates the problem of catastrophic bend loss comprises a core region and a cladding region configured to support and guide the propagation of light in a fundamental transverse mode. The cladding region includes (i) an outer cladding region, (ii) an annular pedestal (or ring) region, (iii) an annular inner trench region, and (iv) an annular outer trench region. The pedestal region and the outer cladding region each have a refractive index relatively close to that of the outer cladding region. In order to suppress HOMs the pedestal region is configured to resonantly couple at least one (unwanted) transverse mode of the core region (other than the fundamental mode) to at least one transverse mode of the pedestal region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of parent application Ser. No. 12/472,522filed on May 27, 2009 now U.S. Pat. No. 8,107,784, which is acontinuation-in-part of grandparent application Ser. No. 12/072,869filed on Feb. 28, 2008 now abandoned, which is in turn acontinuation-in-part of great-grandparent application Ser. No.11/818,780 filed on Jun. 15, 2007 now abandoned. The present applicationalso claims priority from provisional application Ser. No. 61/056,461filed on May 28, 2008 and entitled “Low Bend Loss Fiber with ImprovedFabrication and Tight-Bend Performance.” These applications areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods of making single mode optical fibersand, more particularly, to methods of making such fibers that havereduced bend sensitivity and catastrophic bend loss.

2. Discussion of the Related Art

In contrast with standard single mode optical fiber used, for example,in land line, undersea and metro systems, access fiber, which istypically located closer to the user, includes fiber-to the-home (FTTH),jumper cables, and FTTx fiber (e.g., fiber-to-the-curb, indoor wiring).Access fiber must not only interface in a low loss, reliable way withstandard single mode fiber (SMF), which carries optical signals to thelocation being accessed (e.g., home, business, or other facility), butalso must be relatively insensitive to the effects of bending, which isinherent in many of the access fiber applications.

Thus, in access fiber applications it is highly desirable to have fibersthat combine low bend loss and good compatibility with existinginfrastructure and standards. However, there is an inherent difficultyin achieving low bend loss without sacrificing properties important tocompatibility, especially mode size, splice or connector loss, cutoff,and higher-order mode suppression. Ring-assisted or resonance-assistedfiber (RAF) designs alleviate these difficulties, but many previous RAFdesigns suffer from fabrication and bend range constraints. Fabricationconstraints lead to higher cost and smaller preform size. In particular,the interior region (i.e., excluding the outer cladding) of a RAF has arefractive index profile fabricated using conventional vapor depositiontechniques (e.g., MCVD). The various portions of the interior region(e.g., core, trench, ring/pedestal) have different refractive indices,which can be adjusted by doping with, for example, fluorine or creatinghollow voids to produce a depressed-index region, or germanium toproduce a raised-index region. Due to the large radial extent(cross-sectional area or volume) of the interior-region of a RAFcompared to a conventional single mode fiber, a significant fraction ofthe fiber volume is deposited using the vapor-phase process. Since thedeposition rate of such processes is relatively slow, this type of fibermaterial has relatively low throughput and hence relatively high cost.

Therefore, there is a need for a RAF design that allows at least aportion of the interior-region to be fabricated by a technique otherthan conventional, low-deposition-rate vapor deposition.

In addition to manufacturing cost, current RAFs exhibit an abruptresonant coupling of fundamental mode signal light between the core andthe ring, causing catastrophic optical loss at a critical bend radius,typically in the 3-5 mm range. Yet, recent industry studies haveindicated that tight bend radii (2-4 mm) may occur in some installationsand should be supported.

Therefore, there is also a need for a RAF design that alleviates theproblem of catastrophic bend loss at a critical radius and provides lowbend loss performance over a wider range of bend radii.

BRIEF SUMMARY OF THE INVENTION

In accordance with one aspect of our invention, an optical fiber that isrelatively insensitive to bend loss and alleviates the problem ofcatastrophic bend loss comprises a core region and a cladding regionconfigured to support and guide the propagation of light in afundamental transverse mode, the cladding region including (i) an outercladding region having a refractive index n_(out) less than therefractive index n_(core) of the core region, (ii) an annular pedestal(or ring) region having a refractive index n_(ped), (iii) an annularinner trench region disposed between the core region and the pedestalregion, the inner trench region having a refractive index n_(tri) muchless than that of the pedestal region, and (iv) an annular outer trenchregion disposed between the pedestal region and the outer claddingregion, the outer trench region having a refractive index n_(tro) lessthan that of the pedestal region and relatively close to that of theouter cladding region.

In order to suppress HOMs the pedestal region is configured toresonantly couple at least one transverse mode of the core region (otherthan the fundamental mode) to at least one transverse mode of thepedestal region.

In a preferred embodiment of our fiber, the refractive index and width(or thickness) of the pedestal and outer trench regions are configuredso that the fiber has relatively low bend sensitivity combined with areduced resonant loss peak. In particular, our fiber is configured sothat, at a signal wavelength of approximately 1550 nm, its bend loss isno more than about 0.1 dB/turn at bend radius of 5 mm and is no morethan about 0.02 dB/turn at a bend radius of 10 mm. To this end, thepedestal region is short and wide, whereas the outer trench region isshallow and wide; that is, n_(ped) and n_(tro) are both very close ton_(out), but n_(ped)>n_(tro) and n_(out)>n_(tro). At even tighter radiiin the 2-4 mm range our fiber exhibits comparably low bend loss; forexample, at a bend radius of 3 mm the bend loss is no more than about0.2 dB/turn, with some fibers having a bend loss of less than 0.1dB/turn.

In addition, in one embodiment of our fiber, the core region alsoincludes an inner core region and an annular outer core (or shelf)region surrounding the inner core region. The refractive index of theshelf region is less than that of the inner core region, and thethickness of the shelf region is less than the diameter of the innercore region. According to one aspect of this embodiment, the shelfregion extends radially a distance of less than 9 μm from a longitudinalaxis of the inner core region.

Furthermore, in another embodiment of our fiber, the inner trench regionincludes an annular inner portion and an annular outer (or step) portionsurrounding said inner portion. The refractive index of the step portionis greater than that of the inner portion.

In a preferred embodiment, both of the foregoing features of the coreregion and the inner trench region are incorporated in our fiber.

Fibers designed in accordance with our invention may advantageously beused as access fiber, but may have other applications, such as fibersused in sensors or in vehicles.

In addition, fibers designed in accordance with our invention haveimproved manufacturability in that the ring/pedestal, shallow outertrench and/or the outer cladding regions may be produced usingcommercially available glass tubing, rather than by more expensive,low-deposition-rate techniques.

Thus, another aspect of our invention is a method of making theabove-described RAF, comprising the steps of

-   -   1) providing a first starting tube comprising silica and having        an index n_(ped);    -   2) depositing a multiplicity of down-doped first glass layers on        the inside of the first tube; the first glass layers forming the        deeper-index inner trench region;    -   3) depositing a multiplicity of up-doped second glass layers on        the first layers; the second layers forming the core region;    -   4) collapsing the first tube to form a first rod;    -   5) providing a second tube comprising down-doped glass and        having an index n_(tro);    -   6) providing a third tube comprising silica and having an index        n_(out);    -   7) placing the first rod inside the second tube;    -   8) placing the second tube, with the first rod therein, inside        the third tube; and    -   9) collapsing the third tube and second tube onto the first rod        to form a fiber preform.

After step (9) well known techniques may be used to draw a fiber fromthe preform. By using multiple tubes [steps (1), (5) and (6)] the volumeof silica glass formed by vapor deposition is dramatically reduced, witha concomitant decrease in the fabrication cost.

Variations of the foregoing method utilizing only two starting tubes arealso contemplated by the fabrication techniques of our invention.

Furthermore, alternative methods for overcladding the first rod may beutilized in place of the second or third tubes, and/or alternativemethods for creating the first rod may also be used, such as outsidevapor deposition (OVD) or vapor-phase axial deposition (VAD

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

Our invention, together with its various features and advantages, can bereadily understood from the following more detailed description taken inconjunction with the accompanying drawing, in which:

FIG. 1A is a schematic, cross-sectional view of a Type I RAF, which isdescribed in copending parent application Ser. No. 12/072,869, supra;

FIG. 1B is a schematic graph of the refractive index profile of thefiber of FIG. 1A;

FIG. 1C is a schematic graph of the refractive index profile of aninventive (Type II) RAF in which the outer trench is shallower, inaccordance with one embodiment of the invention described in our parentapplication;

FIG. 1D is a schematic graph of the refractive index profile of Type IIRAF in accordance with one embodiment of the present inventionillustrating a combination of features: a shallow outer trench, a shortpedestal, a core region shelf, and an inner trench region step;

FIG. 2 is a schematic graph of the refractive index profiles of a fiberwith step-index core and an annular pedestal region used to suppresshigher order modes (HOMs); FIG. 2A demonstrates the case for a straightfiber; FIG. 2B for a bent fiber;

FIG. 3 is a schematic graph of the refractive index profile of a Type IIRAF (solid curve I.3), in accordance with one embodiment of ourinvention, compared to the index profiles (dashed curves II.3, III.3) oftwo Type I RAFs as shown in FIG. 1B. Refractive index values are givenrelative to that of the outer cladding region;

FIG. 4 is a graph of simulated HOM confinement loss vs. wavelength forthe three RAFs depicted in FIG. 3;

FIG. 5 is a graph of simulated mode-field diameter (MFD) vs. wavelengthfor the three RAFs depicted in FIG. 3;

FIG. 6 is a graph of simulated bend loss vs. bend radius for the threeRAFs depicted in FIG. 3. The simulation was made at a signal wavelengthof 1550 nm. Similar simulations can be made a longer wavelengths, wherewe would expect the bend losses to be higher, and can be made at shorterwavelengths, where we would expect the bend losses to be lower; and

FIG. 7 is a schematic block diagram of a generalized application of ouraccess fibers.

Various ones of the foregoing figures are shown schematically in thatthey are not drawn to scale and/or, in the interests of simplicity andclarity of illustration, do not include all of the details of an actualoptical fiber or product depicted.

GLOSSARY

Bending: Macro-bending, commonly referred to as simply bending, takesplace when a fiber is bent or curled so that its curvature is relativelyconstant along its length. In contrast, micro-bending takes place whencurvature changes significantly within the adiabatic length scale for aparticular fiber (e.g., along fiber lengths on the order of a millimeteror less). Such micro-bends are formed, for example, in standardmicro-bending tests by pressing the fiber into sand paper.

Center Wavelength: Throughout this discussion references made towavelength are intended to mean the center wavelength of a particularlight emission, it being understood that all such emissions have acharacteristic linewidth that includes a well-known range of wavelengthsabove and below the center wavelength.

Effective Radius: By effective radius we mean the average of the insideand outside radii of an annular region such as a pedestal region or atrench region of a fiber.

Glass Fiber: Optical fiber of the type described herein is typicallymade of glass (e.g., silica) in which the refractive indices of the coreregion and of the cladding region are controlled by the amount and typeof one or more dopants (e.g., P, Al, Ge, F) or by hollow voidsincorporated therein during the fabrication of the fiber, as is wellknown in the art. These refractive indices, as well as thethicknesses/diameters of core/cladding regions, determine importantoperating parameters, as is well known in the art.

Index: The terms index and indices shall mean refractive index andrefractive indices.

Index Profile: The schematic index profiles of FIGS. 1B, 1C, 1D and 3are averages of the actual variations of index that would be observablein an optical fiber. In addition, although various regions of theseprofiles are shown as being rectangular, the boundaries of such regionsneed not be horizontal or vertical; one or more may be slanted, forexample, the region may be trapezoidal.

Mode: The term mode(s) shall mean the transverse mode(s) of anelectromagnetic wave (e.g., signal light).

Mode size: The size of an optical mode is characterized by its effectivearea A_(eff), which is given by:

$A_{eff} = \frac{\left( {\int{{E}^{2}{\mathbb{d}A}}} \right)^{2}}{\int{{E}^{4}{\mathbb{d}A}}}$where E is the transverse spatial envelope of the mode's electric field,and the integrations are understood to be performed over thecross-sectional area of the fiber. When the mode-field shape is close toan axisymmetric (i.e., symmetric about the longitudinal axis of rotationof the fiber) Gaussian function, the mode-field diameter (MFD) is anappropriate metric for the diameter of the mode and may be expressed as:

${MFD} = {2\sqrt{\frac{2{\int{{E}^{2}{\mathbb{d}A}}}}{\int{{\frac{\mathbb{d}E}{\mathbb{d}r}}^{2}{\mathbb{d}A}}}}}$where r is the radial coordinate. When the mode-field shape is exactlyequal to an axisymmetric Gaussian function, then A_(eff)=π×MFD²/4.

Radius/Diameter: Although the use of the terms radius and diameter inthe foregoing (and following) discussion implies that the cross-sectionsof the various regions (e.g., core, pedestal, trench, cladding) arecircular and/or annular, in practice these regions may be non-circular;for example, they may be elliptical, polygonal, irregular or other morecomplex shapes. Nevertheless, as is common in the art, we frequently usethe terms radius and/or diameter for simplicity and clarity.

Resonant Coupling: By the terms resonant or resonantly coupled we meanthat the effective refractive index (n_(eff)) of an unwanted mode (e.g.,a HOM) in the core region is essentially equal to that of a mode in thepedestal region. As explained more fully in the description thatfollows, this phenomenon is used to suppress unwanted HOMs in RAF fibersdesigned in accordance with various embodiments of our invention.

Signal Propagation: Although signal light may actually crisscross thelongitudinal axis as it propagates along a fiber, it is well understoodin the art that the general direction of propagation is fairly stated asbeing along that axis (e.g., axis 16 of FIG. 1A).

Single Mode: References made to light propagation in a single transversemode are intended to include propagation in essentially a single mode;that is, in a practical sense perfect suppression of all other modes maynot always be possible. However, single mode does imply that theintensity of such other modes is either small or insignificant for theintended application.

Suppressed HOM: The degree to which an HOM needs to be suppressed (orcut-off) depends on the particular application. Total or completesuppression is not demanded by many applications, which implies that thecontinued presence of a relatively low intensity HOM may be tolerable.In any event, suppressing HOMs improves system performance by, forexample, reducing total insertion loss, lowering noise in the signalmode, and lowering microbend loss.

Undoped: The term undoped or unintentionally doped means that a regionof a fiber, or a starting tube used to form such a region, contains adopant not intentionally added to the region during fabrication, but theterm does not exclude low levels of background doping that may beinherently incorporated during the fabrication process. Such backgrounddoping levels are low in that they have an insignificant effect on therefractive index of the undoped region.

DETAILED DESCRIPTION OF THE INVENTION

The design of optical access fibers for typical practical applicationsinvolves consideration of three interrelated requirements: (i)relatively low bend loss (i.e., low bend sensitivity) for a bend radiuswithin a predetermined range (e.g., approximately 2-15 mm); (ii)suppression of HOMs (i.e., relatively low cutoff wavelength for theHOM(s) to be suppressed); and (iii) mode-area matching to standard SMF(e.g., good connectorization and/or splicing to standard fiber, such asSMF 28 commercially available from Corning, supra). Below we describefirst what we term Type I RAF designs of the type described in ourparent application Ser. No. 12/072,869, supra. Then, we described TypeII RAFs in accordance with the present invention and compare theirdesign and performance to those of Type I RAFs.

Type I RAF Design—Bend Insensitivity Considerations

With reference now to FIGS. 1A and 1B, an optical fiber 10 in accordancewith parent application Ser. No. 12/072,869, supra, has relatively lowbend loss and, as such, is suitable for a variety of access or sensorfiber applications. Fiber 10 includes a core region of diameter D 12surrounded by a cladding region 14, with the core and cladding regionsbeing configured to support and guide the propagation of signal light(radiation) axially along a longitudinal axis 16 located at essentiallythe center of the core region 12.

Although core region 12 is depicted as having a two-layer profile (i.e.,a profile with two essentially constant or uniform index regions), itcould also have a step-index, multi-step, or graded-index profile.

In accordance with one embodiment of a Type I RAF as described in theaforesaid parent application, the cladding region 14 includes an annularouter cladding region 14.4 (inside edge at r_(out)), an annular,elevated index, pedestal region 14.1 (effective radius at r_(ped);thickness t_(ped)), an annular, depressed index, inner trench region14.2 disposed between the core region 12.1 and the pedestal region 14.1,and an annular, depressed index, outer trench region 14.3 disposedbetween the pedestal region 14.1 and the outer cladding region 14.4.

Other coatings (not shown; e.g., glass or polymer coatings) may surroundthe outer cladding region 14.4, as is well known in the art, forprotection, strength, ease of handling, or other purposes, but do notaffect the optical properties of the fiber.

The refractive index (n_(ped)) of the pedestal region 14.1 is higherthan the refractive index (n_(out)) of the outer cladding region 14.4.In addition, the refractive indices (n_(tri), n_(tro)) of both the innerand outer trench regions are lower than that of the outer claddingregion 14.4; that is, n_(ped)>n_(out), n_(tri)<n_(out), andn_(tro)<n_(out). (Not all of these inequalities are requirements of TypeII RAFs in accordance with the present invention, as discussed infra.)As discussed below, the fiber 10 in general, and the pedestal region14.1 in particular, is configured to suppress preselected (unwanted)HOMs of the core region.

In general, the inner and outer trench regions provide confinement ofthe various fiber modes. The amount or level of confinement for any modenear cutoff can be quantified by the expression (n_(tr)−n_(out))t_(tr),where n_(tr) and t_(tr) are the index and thickness of a trench region.For a Type I RAF, the level of such confinement provided by each of thetrench regions should preferably satisfy the following condition:0.5<[(n _(tri) −n _(out))t _(tri)]/[(n _(tro) −n _(out))t_(tro)]<2.0,  (1a)where t_(tri) and t_(tro) are the thicknesses of the inner and outertrench regions, respectively.

In one embodiment also described in the aforesaid parent application,the core region 12 includes an inner core region 12.1 surroundedradially by an annular outer core region (or shelf region) 12.2. Theindex of the inner core region 12.1 is greater than that of the shelfregion 12.1; that is, n_(core)>n_(shelf). The shelf region has a radialthickness t_(shlf), and its outside edge is positioned at a radiusr_(shelf)=D/2+t_(shlf).

Although Type I RAFs that include the shelf region 12.2 are described inour parent application as being optional, in one embodiment of thepresent invention Type II RAFs with this design feature are preferred.In designs of Type I RAFs where the shelf region is omitted, the coreregion 12 would simply include only the inner core region 12.1, with thethickness of the inner trench region 14.2 being increased by thethickness of the omitted shelf region. As discussed below, in eithercase, the core region 12 is configured to produce a fundamental modeA_(eff) that matches that of a standard SMF.

Bend loss, of course, should be as low as possible. In particular, itshould be less than that of a standard SMF at important operatingwavelengths (e.g., 1300 nm, 1550 nm, and 1650 nm) for any bend radius inthe range of approximately 2-15 mm. To this end, at least one trenchregion 14.2, 14.3 (and preferably both) should provide a total contrastmuch higher than that of a standard SMF. Illustratively, SMF 28 has atotal contrast of about 5×10⁻³ (in units of refractive index). In Type IRAFs described in our parent application, the total contrast of fiber 10is given by|n _(tri) −n _(core)|>0.007, and/or  (1)|n _(tro) −n _(core)|>0.007  (2)Illustratively at least the inner trench-to-core contrast of equation(1) is approximately 0.008-0.020, and preferably both the inner andouter trench-to-core contrasts satisfy this condition for allembodiments except the shallow-trench, Type II RAF embodiments shown inFIGS. 1C, 1D and 3.

In addition, the interface 14.5 (at r_(out)) between the outer claddingregion 14.4 and the outer trench region 14.3 should be at a radius inthe range of approximately 17-23 μm (17-30 μm in the shallow-trenchdesign, infra), and the refractive index of the core and pedestalregions are comparable; that is,|n _(core) −n _(ped)|<0.003  (2a)

The outer trench region 14.3 of Type I RAF 10, as shown in FIG. 1B, isdepicted as being relatively narrow (radial thickness t_(tro)) andrelatively deep (index n_(tro)<<n_(out)). By deep we mean that n_(tro)is more than about 0.0020 below n_(out). The narrowness of outer trenchregion is not critical.

In any of these embodiments the pedestal region (or ring) may be formedin a straight-forward, well-known manner by introducing index-increasingdopants (e.g., Al, Ge, P in silica) into the region during vapordeposition. As illustrated by fiber 10 of FIGS. 1A-1B, in each radialcross-section of the fiber, the ring 14.1 would have a substantiallyuniform index circumferentially. However, as described in the aforesaidparent application, the index of the ring may be rendered non-uniformcircumferentially by means of longitudinally extending, radiallylocalized, well-known features such as index-lowering airholes and/orindex-raising inclusions. As with the core, rings and trenches, thesefeatures may have various cross-sectional shapes including circles,ellipses and polygons. In such designs, the principle of HOM suppressionis still that pedestal modes efficiently couple to unwanted core-guidedmodes when their effective indices are nearly the same. The effectiveindex of pedestal modes can be calculated and index-matched usingstandard methods for fibers with arbitrary cross-section, and so thedesigns are conceptually the same as for the special case of an annulushaving an essentially uniform or constant index. However, the use offeatures with desirable shapes may provide advantages; for example, theymay provide index-matching over a wider wavelength range than ispossible for a uniform-index annulus.

Alternatively, the pedestal region may be a virtual ring; that is, thering need not have a well-defined circumferential (annular) boundariesformed by standard doping during vapor deposition. Instead, the pedestalregion may be formed entirely by a suitably placed array of features:airholes, inclusions, or both.

In a similar fashion, as also described in the aforesaid parentapplication, the inner trench and/or the outer trench may include anarray of suitably spaced airholes that decrease the effective index seenby the propagating signal mode. This approach may be used when thetrenches are formed by vapor deposition or by use of tubes, if the addedcomplexity/expense can be tolerated in customer's intended application.

Finally, as also described in the aforesaid parent application, thefiber may also include multiple, concentric rings, and it may beadvantageous to include a loss region of scattering or absorptioncenters, for example, adjacent an outer ring.

Type I RAF Design—Mode Matching Considerations

Because access fiber applications often entail splicing or otherwisecoupling the access fiber to a standard single mode transmission fiber,it is important that the A_(eff) of the access fiber be matched to thatof the standard SMF (e.g., the standard SMF 28 fiber available fromCorning, supra). In current practice, this requirement means that theaccess fiber should also be effectively single-moded and should have anA_(eff) of about 70-90 μm² at signal wavelengths of approximately 1550nm and an A_(eff) of about 55-70 μm² at signal wavelengths ofapproximately 1300 nm. Typically, for an access fiber core region havinga circular cross-section, the access fiber should have an inner corediameter D of about 8-11 μm approximately.

For simplicity the following exposition, taken from our parentapplication, will focus on the design of Type I RAF 10 of FIGS. 1A-1B.However, it will apparent to those skilled in the art that similarconsiderations apply to the Type II RAF embodiments of FIGS. 1C, 1D and3. The A_(eff) of fiber 10 is controlled primarily by two parameters:the index contrast Δn between the core region 12.1 and the inner trenchregion 14.2; that is, Δn=(n_(core)−n_(tri)) and a radial width or corearea of the core region 12; that is, in the case of a circularcross-section, the diameter D of the core region, but in the case of anon-circular cross-section, the core area. More specifically, for agiven D, when the index contrast is decreased, the confinement of thefundamental mode field decreases, which means that its A_(eff)increases. However, reduced mode confinement means the fiber acts as apoorer waveguide and optical losses increase, particularly when thefiber is subject to sharp bends (e.g., a bend radius of 2-15 mm). On theother hand, for a given Δn, when the diameter of the core region 12increases, the A_(eff) increases (roughly as diameter squared), but thenumber of HOMs supported also increases. In general, the presence ofsignificant energy in HOMs may be undesirable; for example, optical lossincreases if the fiber is subject to micro-bending.

In the alternative embodiment of FIG. 1 in which the outer core region(or shelf) 12.2 is omitted, the total contrast (i.e., n_(core)−n_(tri),or n_(core)−n_(tro), or both) should still satisfy inequalities (1)and/or (2), the core diameter D should be in the range of approximately8 μm≦D≦11 μm,  (3)and the index of the outer cladding region 14.4 should satisfy0.003≦(n _(core) −n _(out))≦0.006  (4)approximately. Note, if this contrast is too high, HOMs tend to beintroduced in the core region, but these HOMs are suppressed using thedesign described in the following section.

An alternative Type I RAF fiber design for meeting the conflictingrequirements of reducing bend loss and matching A_(eff) to that ofstandard SMF is also shown in FIG. 1. More specifically, the core region12 includes a thin, lower index, annular, shelf region 12.2 surroundinginner core region 12.1, as shown in FIG. 1. Shelf region 12.2 allows theA_(eff) to be increased to match that of a standard SMF. The shelfregion 12.2 is relatively thin or narrow; that is, it has a thicknesst_(shlf) that is much less than the diameter D of the inner core region12.1. Illustratively, D>>t_(shlf) and1.0 μm≦t _(shlf)≦4.0 μm  (5)approximately when D=8-11 μm.

In addition, the index n_(shlf) of the shelf region 12.2 is less thanthat of the inner core region 12.1; that is, n_(shlf)<n_(core).Typically, (n_(core)−n_(shlf))<0.007 approximately, which is similar tothe index contrast (0.005) of a standard SMF. Preferably, n_(shlf)satisfies the following inequalities0.003≦(n _(core) −n _(shlf))≦0.007, and  (6)|n _(shlf) −n _(out)|≦0.002  (6a)approximately, and D is the range of approximately 8-10 μm. In thispreferred design, the outside edge of the shelf region should be locatedat a radius r_(shelf)=D/2+t_(shlf), which is less than 9 μm and morethan 5 μm.

The foregoing design details of the core region 12 facilitate not onlymode matching but also HOM suppression, which is discussed in thefollowing section.

In addition, these aspects of inner core region 12.1 and shelf region12.2 are applicable to our Type II RAF designs described infra.

Type I RAF Design—HOM Considerations

In order to suppress HOMs in a Type I RAF, the cladding region 14 offiber 10 includes pedestal region 14.1, which has a higher index n_(ped)than the remainder of the cladding region; that is, the pedestal region14.1 is bounded radially by at least a lower index (n_(tri)) innertrench region 14.2 and, in some embodiments, also by a lower index(n_(tro)) outer trench region 14.3. In addition, it has a higher indexthan the index (n_(out)) of the outer cladding region 14.4. In thediscussion of Type I RAFs that follows, we assume for purposes ofexposition a dual-trench design, with the understanding that similarprinciples apply to the single (inner)-trench design.

The pedestal region 14.1 is configured so that at least one of its(ring) modes resonantly couples with at least one unwanted HOM of thecore region 12. As shown in the simplified index profile of FIG. 2A,preferably HOM 18 (illustratively depicted as an LP₁₁ mode) of the coreregion 12 is resonant with a mode 20 of the pedestal region 14.1,whereas the fundamental mode 22 of the core region is not resonant withany mode of the pedestal region. The mode 20 is typically one of thering modes of pedestal region 14.1 with the highest or nearly thehighest effective index, and the mode 20 is not forbidden by well-knownsymmetry principles from coupling to the HOM 18 of the core region.

By the terms resonant or resonantly coupled we mean that the effectiverefractive index (n_(eff)) of an unwanted mode in the core region isessentially equal to that of a mode in the pedestal region. Thus, then_(eff) 18.1 of the unwanted mode 18 of the core region 12 isessentially equal to the n_(eff) 20.1 of the mode 20 of the pedestalregion 14.1, which allows energy in HOM 18 to transfer or couple (arrow24) from the core region into mode 20 of the pedestal region and fromthere to radiate into the outer cladding region 14.4. (Arrow 26indicates such radiation via leaky cladding modes, which are usuallypresent. Alternatively, this energy may be lost due to absorption,scattering, etc.) After a suitable propagation distance along the fiber,this process of resonant transfer and radiation effectively suppressesHOM 18 in the core region. In contrast, n_(eff) 22.1 of the fundamentalmode 22 of the core region does not correspond to the n_(eff) of anymode in the pedestal region. Consequently, the fundamental mode 22propagates effectively in the core region, and no resonant transfer ofits energy (negated arrow 28) into the pedestal region takes place.

The condition that a core region mode and a pedestal region mode haveessentially equal refractive indices means, for example, that the coreregion HOM index 18.1 and the pedestal region mode index 20.1 are not sodifferent that coupling of light between these modes is significantlyfrustrated. In a preferred embodiment of the invention, the differencebetween indices 18.1 and 20.1 is much less than the difference betweenthe core fundamental mode index 22.1 and the pedestal mode index 20.1.

Proper coupling between a core region mode to be suppressed (i.e., theunwanted mode) and the resonant pedestal region mode should also takeinto account the need to reduce coupling of the latter pedestal modeback into the former core mode.

The fiber 10 should also be configured to allow effective leakage ofunwanted core modes through the pedestal modes. In this regard, see thediscussion above in conjunction with equations (2a) and (2b).

In addition, the coupling between the core region and the pedestalregion should not be so large that the desired (fundamental) core modeis disrupted. On the other hand, the coupling between the core regionand the pedestal region should not be too small that unwanted core modeswill not couple sufficiently to pedestal modes to be suppressed. Next,the leakage rate of the pedestal mode should not be so large thatcoupling between the core and pedestal region is frustrated (i.e.,insufficient). Finally, the leakage rate of the pedestal mode should notbe so small that unwanted core modes will experience too little loss tobe effectively suppressed.

Adherence to these design principles assures that in the core region 12,for example, fundamental mode 22 is effectively propagated, whereas HOM18 (or any other unwanted HOM) is effectively suppressed. The degree towhich the HOM needs to be suppressed (or cut-off) depends on theparticular application. Total or complete suppression is not demanded bymany applications, which implies that the continued presence of arelatively low intensity HOM may be tolerable. In any event, suppressingHOMs improves system performance by, for example, reducing totalinsertion loss, lowering noise in the signal mode, and/or loweringmicrobend loss.

Thus, resonant coupling enables our RAFs to operate in a single mode;e.g., in the fundamental mode 22 (FIG. 2A) of the core region.

When our dual trench, ring fiber is properly designed to effect indexmatching (or resonance) between unwanted HOM core modes and particularring modes, then the slope of core mode and ring mode index curves isnearly the same, especially in the region where they intersect.Consequently, index-matched coupling between the core and ring modes isachieved over a relatively wide wavelength range (i.e. broadband).

The effect of bending on the Type I RAF of FIG. 2A is shown in FIG. 2B.The index profile 4A before bending is skewed as shown by profile 4B,resulting in an increase in n_(eff) 20.1 b of the mode 20 of thepedestal region 14.1. If the bend radius is sufficiently small, thecladding mode 20 may become resonant with the fundamental mode 22 of thecore region 12, as shown by arrow 30. Such resonance woulddisadvantageously and dramatically increase the optical loss of thefundamental core mode 22 (also known as catastrophic bend loss).Accordingly, the pedestal region 14.1 needs to be configured toaccommodate the expected bend radius without causing the fundamentalcore mode 22 to be resonant with any cladding mode, in particular withthe cladding mode 20. The problem of catastrophic bend loss is addressedby the Type II RAF of the present invention, as discussed below.

The foregoing principles of resonant coupling (index matching) may alsobe applied to the suppression of multiple unwanted core modes either byresonantly coupling them to a single, mode of a pedestal region or byresonantly coupling them to different modes of one or more pedestalregions, each core mode being resonant with a separate pedestal mode.

In addition, the foregoing principles of resonant coupling are alsoapplicable to Type II RAFs in accordance with the present invention, asdescribed infra.

Type II RAFs—Shallow Outer Trench

In this section we discuss alternative embodiments (Type II RAFs) of theabove-described RAFs. In Type II RAFs described in our parentapplication, the outer trench region is shallower and wider relative tothe corresponding region of Type I RAFs. One embodiment of such a TypeII RAF is shown in the index profile of FIG. 1C. The outer trench region14.3 c of fiber 10 c is shallower. By shallow we mean that the indexn_(tro) of the outer trench region is relatively close to that of theouter cladding region, and by relatively close we mean n_(tro) is lessthan about 0.002 above or below n_(out). In addition, FIG. 1C alsodepicts the shallow outer trench region 14.3 c as being wider that theinner trench region 14.2 c, but, as with the design of FIG. 1B, thewidth (thickness) t_(tro) of the outer trench region 14.3 c is notcritical.

At first blush it appears that this shallow-trench design contravenesthe design principle defined by equation (1a); that is, the level ofconfinement provided by each of the trenches should be roughly the same.In a large portion of the design space, combining a highly confining(deep) inner trench with a much less confining (shallower) outer trenchgives relatively poor HOM suppression, in part because the ring modes ofthe pedestal region 14.1 become too lossy and too isolated from the coremodes, which interferes with the very purpose (HOM suppression) of thepedestal region, discussed supra. However, we have found a design spacewhere good performance is obtained despite the having two trenches withvery dissimilar levels of confinement; that is, where:[(n _(tri) −n _(out))t _(tri)]/[(n _(tro) −n _(out))t _(tro)]>2.0,  (2b)e.g., where the left hand side of equation (2b) is illustratively in therange of about 5-9. In this design space, pedestal modes are not wellconfined to the pedestal region; that is, they extend into the outertrench region and have large losses due to tunneling into the outercladding. Poor confinement of the pedestal mode tends to degrade the HOMsuppressing performance of these fibers, but this disadvantage isbalanced by the enhanced bend loss performance of these designs. Whenbent, the pedestal modes of the fiber become extremely lossy making thefiber immune to the catastrophic bend loss effect discussed supra inconjunction with FIG. 2B.

In the Type II index profile of the RAF of FIG. 1C, as well as in theType I RAFs of FIGS. 1A-1B, the pedestal/ring region 14.1 c is depictedas having an index that is significantly greater than that of the outercladding region 14.4 c. However, we have found that this particularfeature is not essential in shallow outer trench embodiments of ourinvention, as shown, for example, in Type II RAF 10 d of FIG. 1D. Here,the refractive index of the pedestal region 14.1 d (n_(ped)) isrelatively close to that of the outer cladding region 14.4 d (n_(oc))but is still greater than the refractive indices of the shallow outertrench region 14.3 d (n_(tro)) and the deeper inner trench region 14.2 d(n_(tri), n_(step)). The refractive index of the shallow outer trenchregion 14.3 d preferably is relatively close to that of the outer coreregion 14.4 d, as mentioned previously.

In one embodiment, the core region 12 d of our Type II RAF 10 d alsoincludes an inner core region 12.1 d and an outer core or shelf region12.2 d, as described above in conjunction with Type I RAFs. As discussedpreviously, the radius of the shelf (r_(shelf)) is preferably less than9 μm and greater than about 5 μm in order to reduce bend sensitivity. Inanother embodiment, the inner trench region 14.2 d includes a deeperinner portion 14.21 d and a shallower outer portion or step 14.22 d;that is, the refractive index of the shallower step 14.22 d (n_(step))is greater (less negative) than that of the deeper inner portion 14.21 d(n_(step)). In a preferred embodiment, as depicted in FIG. 1D, both ofthe latter features (shelf 12.2 d and step 14.22 d) are included in thefiber design. Illustratively, as shown in Table I below, the radius ofthe inner core (D/2) is about 2.5 times the thickness of the shelfregion 12.2 d (t_(shelf)), and the thickness of the inner portion 14.21d of the inner trench region 14.2 d is about 2 times the thickness ofthe step 14.22 d, but neither of these ratios is critical. Differentratios can be used with appropriate, readily calculated, adjustments inother design parameters.

Type II RAFs of our preferred design have exceptional bend lossperformance characteristics—at a signal wavelength of approximately 1550nm, they exhibit bend loss of no more than about 0.1 dB/turn at bendradius of 5 mm and no more than about 0.02 dB/turn at a bend radius of10 mm.

Type II RAFs—Performance & Design Principles

In order to demonstrate the enhanced performance of our Type II RAF, wecompare it with two designs of Type I RAFs of the type described in ourparent application. The index profiles of the three RAFs are shown inFIG. 3, where solid curve I.3 is the profile of a Type II RAF (with adeep inner trench and shallow outer trench) in accordance with thepresent invention, dashed curve II.3 is the profile of a Type I RAF(with medium contrast inner and outer trenches), and dot-dashed curveIII.3 is the profile of another Type I RAF (with a high contrast, deepinner trench and a medium contrast outer trench).

The specific design parameters of these fibers are listed in Table Ibelow:

TABLE I Type II RAF Type I RAF Type I RAF (FIG. 3; (FIG. 3; (FIG. 3;Design Parameter profile I.3) profile II.3) profile III.3) Core:n_(core)  4.2 × 10⁻³  3.9 × 10⁻³  4.1 × 10⁻³ D 9.2 μm 8.7 μm 8.9 μmn_(shlf)  0.7 × 10⁻³ −0.4 × 10⁻³ −1.0 × 10⁻³ t_(shlf) 1.8 μm 2.2 μm 2.3μm r_(shlf) 6.4 μm 6.6 μm 6.7 μm Trench: n_(tri) −9.6 × 10⁻³ −5.7 × 10⁻³−9.7 × 10⁻³ t_(tri) 5.0 μm 4.7 μm 6.7 μm n_(step) −6.0 × 10⁻³ notapplicable not applicable t_(step) 2.4 μm not applicable not applicablePedestal: n_(ped) 0  3.9 × 10⁻³  3.6 × 10⁻³ t_(ped) 7.3 μm 2.6 μm 3.0 μmr_(ped) 17.5 μm  12.6 μm  12.2 μm  Trench: n_(tro) −0.8 × 10⁻³ −5.7 ×10⁻³ −6.0 × 10⁻³ t_(tro) 7.3 μm 7.2 μm 8.6 μm Outer clad: n_(out) 0 0 0r_(out) 28.5 μm  21.0 μm  22.3 μm 

The calculated performance of the three RAFs is summarized in FIG. 4(HOM confinement loss vs wavelength, which is related to cutoff andMPI), FIG. 5 (MFD vs wavelength, which is related to splicing andconnectorization loss), and FIG. 6 (bend loss vs bend diameter, whichquantifies how sensitive the fiber is to bending). The tradeoffs amongthese three parameters are discussed below.

Bend Loss: Comparing the two Type I RAFs, FIGS. 4-6 demonstrate that theType I RAF having the deeper-inner trench (curve III.3; FIG. 3) has ˜1/10th the bend loss (curve III.6 vs. curve II.6; FIG. 6) of the Type IRAF having medium contrast trenches (curve II.3; FIG. 3) for all radii.However, the deeper inner trench, Type I RAF has lower MFD (curve III.5;FIG. 5) and more persistent HOMs (curve III.4; FIG. 4). By morepersistent we mean that the confinement loss of the HOMs is lower.

On the other hand, both Type I RAFs have very high bend loss for bendradii significantly below about 5 mm (curves II.6, III.6; FIG. 6) due tothe fundamental-mode resonant coupling around the critical radius(resonant peaks 6.2, 6.3 at r_(crit)˜4 mm). In contrast, theshallow-trench Type II RAF (curve I.3; FIG. 3) has lower bend loss forbend radii as low as 2.5 mm (curve I.6; FIG. 6). In addition, the TypeII RAF has a larger critical radius (r_(crit)˜7.5 mm), but theassociated fundamental loss peak 6.1 is much smaller, demonstrating thatthis detrimental peak can be rendered insignificant in Type II RAFdesigns in accordance with the present invention.

The critical radius of our Type II RAF is larger than that of the Type IRAF designs because the pedestal/ring is positioned at a larger radiusthan the rings of the Type I RAFs.

For the Type II RAF bend loss (FIG. 6) is better in some importantbending conditions (e.g., for tighter bends less than ˜4.5 mm radius)and worse at others (bends greater than ˜4.5 mm radius). Resonantcoupling essentially gives us the ability to control the shape of thisbend loss curve, so that optimal performance is achieved at the mostimportant bend radii for a particular application. Nevertheless, ourType II RAFs have utility over a relatively broad range of bend radii ofapproximately 2-15 mm. (In this context, we consider our simulations andexperimental results at a bend radius of 2.5 mm to be applicable to abend radius of approximately 2 mm.)

Catastrophic Bend Loss: Our Type II RAF design has distinct advantagesover the Type I RAFs. More specifically, the performance of a Type I RAF(curve II.3, FIG. 3) is characterized by catastrophic bend loss; thatis, a resonant loss peaks 6.2, 6.3 (curves II.6, III.6; FIG. 6), whichoccur at a critical bend radius (r_(crit)˜4 mm in this illustration). Inaddition, when the bend radius is smaller than about 6.5 mm, forexample, the bend loss increases dramatically to peak loss 6.2, 6.3 and,in addition, remains relatively high at radii below r_(crit) down toabout 2.5 mm. In contrast, the bend loss of the Type II RAF (curve I.6,FIG. 6) exhibits no comparable resonant loss peak over the same range ofradii. (The Type II RAF exhibits only a very shallow, insignificant peak6.1 around 7.5 mm.) Moreover, the Type II RAF also exhibits much lowerbend loss at bend radii between approximately 2.5-4 mm.

Our analysis indicates that the absence of a resonant loss peak in ourType II RAFs is a result of the design feature whereby the index of theouter trench region is relatively close to the index of the outercladding region. In these shallow-trench designs, the outer trenchregion provides much less confinement of the pedestal/ring mode than theinner trench region, thereby reducing the resonant coupling between thecore region and the pedestal/ring region. This design substantiallymitigates the problem of high bend loss at tight bends near the criticalradius.

HOM Suppression: Comparing our Type II RAF (curve I.3, FIG. 3) to theType I RAF with deep inner trench (curve III.3), we see that theyexhibit similar HOM confinement loss performance (curves I.4 vs. III.4;FIG. 4), but the Type II RAF has significantly better (higher) MFD(curve I.5 vs. III.5; FIG. 5). In contrast, the medium trench Type I RAFdesign (curve II.3, FIG. 3) yields higher MFD (curve II.5; FIG. 5) andless persistent HOMs (curve II.4; FIG. 4), but suffers from the highestand most detrimental resonant loss peak (6.3; FIG. 6) of all threefibers.

Cutoff: In conventional single mode fibers, it is highly desirable forany HOMs to be effectively cutoff to minimize signal interference.However, for many applications benefiting from highly bend-insensitivefibers, mode-coupling perturbations are relatively weak or relativelyfar apart, and the applications can be much more tolerant to thepresence of light in otherwise unwanted HOMs. For such applications,HOMs can be considered effectively cutoff even with a lower HOMconfinement loss than would be required if mode coupling perturbationswere stronger or more closely spaced. A lower requirement on HOMconfinement loss allows greater flexibility in fiber design. Thus, forexample, HOM loss curves III.4 and I.4 may indicate effective cutoffwavelengths around 1300 nm for many applications of interest.

Design: Since the short wide pedestals/rings of our Type II RAFs have alarge effective radius, the critical radius for resonant coupling of thefundamental core mode is larger than for relatively taller narrowerpedestals. Since the critical radius is more likely to fall within therange desired for commercial use and installations, it is important thatsuch short, wide pedestals be used in designs that suppress catastrophicloss. In other words, short pedestals should be paired with shallowtrenches, as shown by the index profiles of FIG. 3 (curve I.3) and FIG.1D.

Illustratively, the total thickness (r_(out), FIG. 1D) of the core,trench and pedestal regions is nearly 30 μm (FIG. 3), the totalthickness of the trench and pedestal regions exceeds about 20 μm, andthe thickness of each of the trench and pedestal regions is about 6-8μm. Thus, as illustrated in the index profile I.3 (FIG. 3), the innercore diameter is D-9 microns, the core and shelf region have a combineddiameter of about 13 μm, and each of the inner trench, pedestal andouter trench regions is about 7.5 μm thick. (See also Table I.)

Type II RAFs—Fabrication Considerations

As noted in the aforesaid parent application, the shallower trenchregion 14.3 c (FIG. 1C) is expected to improve manufacturability offiber 10 c because the outer trench could be created using a glass withrelatively low levels of down-doping, such as a down-doped substratetube [e.g., a F-doped silica (glass) substrate (or starting) tube]rather than a vapor-deposited glass. Use of a substrate tube wouldreduce the amount of vapor-deposited glass required to fabricate thefiber 10 c, thereby reducing manufacturing cost.

In this section, we expand on that theme. In particular, we recognizethat the optical performance of an RAF is sensitive to manufacturingvariations in the refractive index profile of the fiber. For example,small changes in the index, location or width (thickness) of the trenchregions and/or pedestal region can have a large impact oncharacteristics like bend loss, MFD, cutoff wavelength and dispersion.It is desirable, therefore, to minimize this sensitivity to improvemanufacturing yield.

Of particular interest is the impact of the pedestal regioncharacteristics. Pedestal regions with high refractive index (e.g., FIG.1C; and FIG. 3, curves II.3 and III.3) compared to the outer claddingregion are typically narrow in desirable Type I RAF designs. Conversely,in a Type II RAF similar optical performance can be obtained using apedestal region with an index closer to the outer cladding index but ofincreased width or thickness (FIG. 1D; FIG. 3, curve I.3). Althoughoptical performance may be similar, designs with tall thin pedestalregions are significantly more sensitive to fractional variations in theparameters of the index profile than are designs with short widepedestal regions. For example, although a 10% manufacturing variation inpedestal region width (thickness) may not alter bending performance forshort wide pedestal regions, such a variation may have significantimpact for comparable designs using tall thin pedestal regions.

Just as use of shallow trench regions facilitates the use of glass tubesor glass produced with higher deposition rate, so too can the use ofshort pedestal regions. In low loss designs, both the pedestal and outertrench regions can be formed using commercially-available glass tubing.For example, the pedestal region can be formed from be a pure (i.e., orunintentionally doped) silica substrate or starting tube inside of whichis deposited (e.g., by MCVD) the inner trench and core regions, whilethe outer trench region can be created by overcladding this MCVD corerod with a slightly down-doped starting tube, followed by overcladdingwith another pure silica starting tube. Wide pedestal and outer trenchregions are desirable if they are produced from such tubes.

The available types of doped, high-silica tubes compatible with opticalfiber preform manufacture are currently limited to low index contrastcompared to pure silica, and typically they have a refractive index lessthan that of pure silica. For example, the typical index ofcommercially-available, low-cost tubing ranges from around 2×10⁻³ belowpure silica up to the index of pure silica. Such tubes are doped withfluorine to produce a lower index than pure silica.

Useful high-deposition-rate techniques include external soot processesand glass grain or sand processes. [See, for example, C. Pedrido, WO2005/102946 (2005).] In these techniques it is desirable to maintain lowindex contrast compared to pure silica and wide deposition regions toimprove both speed and manufacturing cost.

In addition, while some of our Type I RAFs have had high-contrastdeposited regions extending beyond 20 μm, some of our Type II RAFs mayhave deposited regions confined within, for example, a 14 μm radius,thereby reducing the volume of deposited glass by more than a factor oftwo.

Taking into account these considerations, Method I of making the Type IIRAFs of the type described above with reference to FIGS. 3-6 comprisesthe following process steps:

-   -   1) providing a first starting tube (also known as a substrate        tube) comprising silica and having an index n_(ped);    -   2) depositing a multiplicity of down-doped first glass layers on        the inside of the first tube; the first glass layers forming the        deeper-index inner trench region;    -   3) depositing a multiplicity of up-doped second glass layers on        the first layers; the second layers forming the raised-index        core region;    -   4) collapsing the first tube to form a first rod;    -   5) providing a second tube comprising down-doped glass and        having an index n_(tro);    -   6) providing a third tube comprising silica and having an index        n_(out);    -   7) placing the first rod inside the second tube;    -   8) placing the second tube, with the first rod therein, inside        the third tube; and    -   9) collapsing the third tube and second tube onto the first rod        to form a fiber preform.        After step (9) techniques well known to those skilled in the        optical fiber art may be used to draw a fiber from the preform.        In addition, each of the foregoing steps may in practice include        multiple sub-steps. Thus, for example, depositing step (3) may        include the sub-steps of (3a) depositing a first multiplicity of        up-doped layers on the second layers to form the outer core (or        shelf) region 12.2 d (FIG. 1D) and then (3b) depositing a second        multiplicity of up-doped layers on the shelf region 12.2 d to        form the inner core region 12.1 d. Likewise, depositing step (4)        may include the sub-steps of (4a) depositing a first        multiplicity of down-doped layers on the first layers to form        the shallower, outer (or step) portion 14.22 d of the deep inner        trench region 14.2 d (FIG. 3) and then (4b) depositing a second        multiplicity of down-doped layers on the step portion 14.22 d to        form the inner portion 14.21 d of the deep inner trench region.

By using multiple tubes [steps (1), (5) and (6)] the volume of silicaglass formed by relatively low rate vapor deposition is dramaticallyreduced, with a concomitant decrease in the fabrication cost. A similarbenefit is obtained when substituting the aforementioned external sootor grain processes in place of the second or third tubes in order tooverclad the first rod. More specifically, such overcladdings may beformed by deposition of silica-based soot from a flame or by use ofglass grain sintered into clear glass. Such methods can be practiced insuch a fashion as to create annular regions of different refractiveindex (e.g. n_(tro) and n_(out)) with higher rates of deposition thanused for the core or inner cladding regions.

Furthermore, alternative methods for creating the first rod may also beused, such as outside vapor deposition (OVD) or vapor-phase axialdeposition (VAD). Since fabrication of the inner regions of a RAF bythese methods is typically considerably slower than for outer claddingdeposition, it is beneficial to reduce the radial extent of glassdeposited using these alternate methods. Indeed, any combination ofvapor phase, tube, soot or grain processes may be used to create theinventive index structure so long as the radial extent of materialproduced from a relatively slow deposition process is reduced.

In Method I, we prefer the fiber design in which the deep inner trench14.2 d (FIG. 1D) includes a deeper inner portion 14.21 d and a shallowerouter portion (or step) 14.22 d. The presence of the step 14.22 d hascertain fabrication advantages. More specifically, in step (2) of MethodI, we have found that the composition of the substrate tube, which ispurchased from commercial sources, may adversely affect the quality ofthe first layers deposited thereon depending on the deposition techniqueused.

In particular, when using a double-pass technique to form each layer ofthe deep inner trench 14.2 d (deposit soot in the first pass; sintersoot in the second pass) directly on the substrate tube, we have foundthat high quality silica layers are difficult to obtain. On the otherhand, we have overcome this problem by forming the shallower outertrench portion 14.22 d using a single-pass deposition technique (e.g.,standard MCVD); that is, each layer of outer portion 14.22 d isdeposited as a F-containing silica layer using SiF₄ in a single pass ofthe torch. However, this technique is capable of producing a maximumnegative refractive index only about −7×10⁻³, which is insufficient forthe deeper inner portion 14.21 d, which typically requires a morenegative index of about −10×10⁻³. To attain the requisite index of thedeeper inner portion, we use a double-pass deposition technique; thatis, each layer of inner portion 14.22 d is deposited in two passes ofthe torch: on a first pass, a silica-soot layer is deposited, and then,on a second pass, the soot layer is sintered in the presence of SiF₄ toform a F-containing silica layer of the inner portion 14.21 d.

More generally, inner trench region deposition techniques depend on thedesired refractive index (n_(tri)) desired, as follows:

-   -   (a) If 0<n_(tri)≦−4×10⁻³, then the inner trench region comprises        multiple silica layers each formed by single-pass deposition of        silica in the presence of SF₆;    -   (b) If −4×10⁻³≦n_(tri)≦−7×10⁻³, then the inner trench region        comprises multiple silica layers each formed by single-pass        deposition of silica in the presence of SiF₄; or    -   (c) If −7×10³≦n_(tri)≦−11×10⁻³, then the inner trench region        comprises multiple silica layers each formed by double-pass        deposition of silica by first deposing soot and tehn sintering        the soot in the presence of SiF₄, as described above; or    -   (d) Combinations of these techniques; in particular, single-pass        depositions in the presence of both SiF₄ and SF₆.    -   (e) The deep inner trench of (a), (b), (c) or (d) can be        produced from alternate methods, such as sintering        externally-deposited soot in an atmosphere containing high        partial pressure of SiF₄, or using small hollow voids to reduce        the average refractive index of the glass region.        Although the above-described three-tube method of fabrication is        preferred because it reduces the volume of low-deposition-rate        glass deposited and hence the cost of manufacturing the fiber,        there may be applications in which a somewhat higher cost can be        tolerated. In such cases, an alternative approach would be (i)        to deposit the pedestal region, the inner trench region and the        core region before the collapsing step (4) that forms the first        rod and (ii) to use only two starting tubes, one to form the        outer trench region of step (5) above and the other to form the        outer cladding region of step (6) above.

Accordingly, this alternative two-tube Method H of fabricating our TypeII RAF comprises the following steps:

-   -   1) providing a first starting tube (also known as a substrate        tube) comprising silica and having an index n_(tro);    -   2) depositing a multiplicity of first glass layers on the inside        of the first tube; the first layers forming the pedestal region;    -   3) depositing a multiplicity of down-doped second glass layers        on the first layers; the second glass layers forming the        deeper-index inner trench region;    -   4) depositing a multiplicity of up-doped third glass layers on        the second layers; the third layers forming the raised-index        core region;    -   5) collapsing the first tube to form a first rod;    -   6) providing a second tube comprising silica and having an index        N_(out);    -   7) placing the first rod inside the second tube;    -   8) collapsing the second tube to form a fiber preform.        As before, after step (8) techniques well known to those skilled        in the optical fiber art may be used to draw a fiber from the        preform, and, as with the preferred embodiment, each of the        above processing steps may include a plurality of sub-steps.

By using multiple tubes [steps (1) and (6)] the volume of silica glassformed by relatively low rate vapor deposition is reduced compared withType I RAFs, with a concomitant decrease in the fabrication cost. Thereduction, however, is not as great as that achieved with the preferredmethod. As before, a similar benefit is obtained when substitutingexternal soot or grain processes in place of the second tube, while thecore rod may be produced using other well known methods for producingthe material of the inner region of the RAF.

When using Method II, where the first layers deposited on the substratetube do not form a low index trench region, but rather form a higherindex pedestal region, it is unnecessary to utilize a single-passdeposition process followed by a double-pass deposition process.Instead, the pedestal region may be formed by single-pass MCVDdeposition, and the entire deep inner trench region may be formed by adouble-pass technique so that the region exhibits an index of about−7×10⁻³ or less, as in technique (c) above. Alternatively, the trenchregion may be formed using one of techniques (a), (b) (d) or (e) abovedepending on the desired index.

Experimental Results

This example describes Type II RAFs of the type shown by the indexprofile I.3 of FIG. 3. Various materials, dimensions and operatingconditions are provided by way of illustration only and, unlessotherwise expressly stated, are not intended to limit the scope of theinvention.

We fabricated these Type II RAFs using the above-described 3-tubepreferred method and well known MCVD for the silica layer depositionsteps. All tubes were obtained from commercial sources. The tubes forthe pedestal region and the outer cladding region comprised undopedsilica, whereas the tube for the outer trench region comprised F-dopedsilica and had a refractive index 2×10⁻³ below that of the outercladding.

The MCVD silica layers were doped with Ge in inner core region 12.1 d(FIG. 1D), Ge in annular outer core region 12.2 d, and F in inner trenchregion 14.2 d. As noted above, the outer trench region 14.3 d was formedby a F-doped tube, and the outer cladding region 14.4 d was formed by anundoped tube. The as-drawn fibers had the index contrast profiles shownby curve I.3 of FIG. 3.

We measured bend loss vs. bend diameter of our Type II RAFs at a signalwavelength of 1550 nm and confirmed that the data for each fiberconformed well to curve I.6 (FIG. 6). In particular, these fiberssatisfied the following criterion: the bend loss was no more than about0.1 dB/turn at a bend radius of 5 mm and was no more than about 0.01dB/turn at a bend radius of 10 mm. In fact, their performance was evenbetter; that is, our fibers exhibited bend loss of about 0.06-0.08dB/turn at a bend radius of 5 mm and bend loss of about 0.006-0.009dB/turn at a bend radius of 10 mm.

In addition, we estimated the cutoff from measurements to be around 1262nm for 22 m lengths of fiber. The mode field diameter at 1310 nmwavelength was measured to be 8.8±0.1 μm.

Another group of fibers fabricated from multiple preforms in accordancewith the principles of our invention exhibited bend loss and cablecutoff properties as follows:

Cable cutoff: 1200-1250 nm;

Bend loss: 0.04-0.08 dB/turn at a bend radius of 5 mm and a wavelengthof 1550 nm;

Bend loss: 0.005-0.02 dB/turn at a bend radius of 10 mm and a wavelengthof 1550 nm.

However, we prefer to utilize the more conservative criterion of 0.02dB/turn at a bend radius of 10 mm to allow for inevitable manufacturingvariations that can adversely affect the bend loss performance offibers. In addition, as noted earlier, at even tighter radii in the 2-4mm range our fiber exhibits comparably low bend loss; for example, at abend radius of 3 mm the bend loss is no more than about 0.2 dB/turn,with some fibers having a bend loss of less than 0.1 dB/turn.

It is to be understood that the above-described arrangements are merelyillustrative of the many possible specific embodiments that can bedevised to represent application of the principles of the invention.Numerous and varied other arrangements can be devised in accordance withthese principles by those skilled in the art without departing from thespirit and scope of the invention.

In particular, although we described above how various fiber dimensionsaffect ring-mode confinement losses, and hence reduce the amount ofoptical energy coupled back from the pedestal region into the coreregion, it will be apparent to those skilled in the art that there areother ways to accomplish the same result; e.g., by use of absorption,scattering, fiber bends, mode coupling, or gain. Moreover, thesetechniques may be used separately or in combination with one another.

In addition, an illustrative, highly generalized application of ouraccess and/or FTTH fibers is shown in FIG. 7. Here, an input fiber(e.g., a standard SMF 70) carries an optical signal from a source 72(e.g., a transmission system) to a facility 74 (e.g., a building housinga business or home). Illustratively, SMF 70 is spliced to an accessfiber 76, a Type II RAF in accordance with our invention. Fiber 76carries the signal to a utilization device or apparatus 78 locatedwithin or associated with the facility. SMF 70 and access fiber 76 areillustratively spliced to one another at a connection box 77, which istypically located on an interior or exterior wall 74.1 of facility 74.However, the connection box could be located elsewhere within thefacility or outside it. In either case, access fiber 76 typically doesnot have a straight line path to utilization apparatus 78. Rather, itoften has to navigate around one or more obstacles 79, which means thatit frequently has at least one curved segment or section 76.1. Asdescribed previously, such curved sections may have a tight bends inwhich the fiber bend radius is 2-15 mm, approximately. The mode-matchingfeatures of our access fibers permit them to be efficiently spliced toSMF and at the same to be bent around obstacles without experiencingexcessive bend loss. Alternatively, SMF 70 may be an output fiber orboth an input and an output fiber. Therefore, in general SMF 70 may bereferred to as an input/output fiber.

Of course, those skilled in the art will readily recognize that thecurved segment or section 76.1 could also be located outside thefacility 74.

Finally, although we have emphasized the use of our fibers in accessapplications, it will also be apparent to those skilled in the art thatthe reduced bend sensitivity of these fibers renders them attractive foruse in, for example, sensors or vehicles (e.g., automobiles, airplanes,trains, boats).

1. A method of fabricating an optical fiber wherein said fiber comprisesa core region having a longitudinal axis, and a cladding regionsurrounding said core region, said core and cladding regions configuredto support and guide the propagation of signal light in a fundamentaltransverse mode in said core region in the direction of said axis, saidcladding region including an outer cladding region having a refractiveindex (n_(out)) less than that of said core region, a pedestal regionhaving a refractive index (n_(ped)) approximately equal to that of saidouter cladding region, an annular inner trench region disposed betweensaid core region and pedestal region, an annular outer trench regiondisposed between said pedestal region and said outer cladding region,said pedestal region having a refractive index greater than therefractive index (n_(tri)) of said inner trench region and therefractive index (n_(tro)) of said outer trench region, and saidpedestal region being configured to resonantly couple at least onetransverse mode of said core region, other than said fundamental mode,to at least one transverse mode of said pedestal region, and whereinsaid method includes the steps of: 1) providing a first starting tubecomprising silica and having an index n_(tro); 2) depositing amultiplicity of first glass layers on the inside of the first tube; thefirst layers forming the pedestal region; 3) depositing a multiplicityof down-doped second glass layers on the first layers; the second glasslayers forming the deeper-index inner trench region; 4) depositing amultiplicity of up-doped third glass layers on the second layers; thethird layers forming the core region; 5) collapsing the first tube toform a first rod; 6) providing a second tube comprising silica andhaving an index n_(out); 7) placing the first rod inside the secondtube; 8) collapsing the second tube to form a fiber preform.
 2. Themethod of claim 1, further including the step of drawing said fiber fromsaid preform.
 3. A method of fabricating an optical fiber wherein saidfiber comprises a core region having a longitudinal axis, and a claddingregion surrounding said core region, said core and cladding regionsconfigured to support and guide the propagation of signal light in afundamental transverse mode in said core region in the direction of saidaxis, said cladding region including an outer cladding region having arefractive index (n_(out)) less than that of said core region, apedestal region having a refractive index (n_(ped)) approximately equalto that of said outer cladding region, an annular inner trench regiondisposed between said core region and pedestal region, an annular outertrench region disposed between said pedestal region and said outercladding region, said pedestal region having a refractive index greaterthan the refractive index (n_(tri)) of said inner trench region and therefractive index (n_(tro)) of said outer trench region, and saidpedestal region being configured to resonantly couple at least one othertransverse mode of said core region to at least one transverse mode ofsaid pedestal region, and wherein said method includes the steps of: 1)providing a first starting tube comprising silica and having an indexn_(ped); 2) depositing a multiplicity of down-doped first glass layerson the inside of the first tube; the first glass layers forming thedeeper-index inner trench region; 3) depositing a multiplicity ofup-doped second glass layers on the first layers; the second layersforming the core region; 4) collapsing the first tube to form a firstrod; 5) providing a second tube comprising down-doped silica and havingan index n_(tro); 6) placing the first rod inside the second tube; 7)providing a third tube comprising silica and having an index n_(out); 8)placing the first rod inside the second tube; 9) placing the second rodinside the third tube; and 10) collapsing the third tube and second tubeonto the first rod to form a fiber preform.
 4. The method of claim 3,further including the step of drawing said fiber from said preform. 5.The method of claim 3, wherein said inner trench region includes ashallower outer portion formed on said first tube a deeper inner portionformed on said outer portion, each of said portions comprising multiplesilica layers, and wherein step (2) includes the sub-steps of: (2a)forming each layer of said shallower outer portion in a single-passdeposition of a F-containing silica layer; and (2b) forming each layerof said deeper inner portion in a double-pass deposition in which, in afirst pass, a layer of silica soot is deposited and then, in a secondpass, the soot is sintered in the presence of SiF₄.